1. Field of the Invention
The present invention relates to a novel pad structure for electrical contact. More particularly, the present invention relates to an exposed contact pad/finger or a bond pad structure on a circuit board or package substrate, and fabrication methods thereof. The invention provides advantages including lower manufacturing cost and improved resistance to grinding, scratching or corrosion.
2. Description of the Prior Art
A circuit board or a package substrate is constructed with dense circuit patterns and intensive fine copper interconnecting traces that are electrically connected to a number of contact pads or bond pads distributed on the surface of the package substrate or circuit board for the transmission of electronic signals or power. On the top surface of the contact pads or bond pads such as wire-bond fingers or solder bond pads, a Ni/Au layer is typically formed to ensure that the these bond pads are in excellent electrical coupling with a circuit of a chip. Furthermore, the Ni/Au layer can prevent the contact pads or bond pads from oxidation thereby improving the electrical interconnection performance of the pads.
As known in the art, electroplating and chemical plating methods have long been used to form the Ni/Au layer on the contact pads or bond pads. Typically, the Ni/Au layer on the wire-bond fingers is formed by electroplating method. The gold layer formed by electroplating method is structurally dense and is able to prevent the underlying nickel layer from corrosion. In addition, the electroplated gold layer has better bondability. Nevertheless, a thicker gold layer, say 0.4-0.9 μm, is required when the gold layer is formed by electroplating method. Therefore the manufacturing cost is higher.
The Ni/Au layer on the surface mount device (SMD) is typically formed by chemical plating method and has a thickness of gold ranging between 0.1-0.2 μm, therefore, the manufacturing cost is lower. However, the so-called black pad problem occurs when using chemical plating method, which may cause poor bondability between the solder balls and the pads.
This industry has developed chemical plated Ni/Pd/Au layer to thereby broadly incorporate the Ni/Au layer formed by chemical plating method into the technology field of wire bonding. To improve the corrosion resistance of the chemical plated nickel layer, an intermediate palladium layer is plated on chemical plated nickel layer. However, the improvement of the corrosion resistance corrosion of the chemical plated nickel layer is not adequate. Besides, palladium is also expensive so the manufacturing cost is still high.
Further, it is known that the Ni/Au layer formed by the prior art methods has insufficient resistance to grinding or scratching. In some circumstances, it is also required that the above-mentioned the exposed wire-bond fingers or solder bond pads on the circuit board or package substrate should have better resistance to grinding or scratching in some circumstances in addition to corrosion resistance.